Betta turning blue

[cherisec]

I bought a Betta about a month ago that was metallic blue with white blotches and red in its fins. It is now losing its white and is going completely blue. Is my fish becoming stressed and unhealthy or is it possible that he was unhealthy when I bought him? He was being kept in a tiny glass cube. I've got him in a 2 gallon tank now and am doing frequent water changes because I'm struggling a bit with algae. I've attached a photo of what he looked like when I bought him.

 

[SaraB]

Bettas change colour as they mature. I had a female white with red fins betta turn completely pink/red as she aged. I wouldn't worry about it.